Add note to INSTALL guide about components that may move to opentelemetry-cpp-contrib

[dbarker]

Discussed in the SIG meeting on 5/14/25:

• The INSTALL guide now has a table of components in the cmake package. This is new for the 1.21.0 release
• We should add a note about the components that may move to opentelemetry-cpp-contrib in the future.
  • etw exporter #1423
  • elasticsearch exporter #1424